Cycles toonshader vs Blender Internal shaders
At this point, it's really most dependent on which material system you're more comfortable with. With Cycles' node system and OSL, you can achieve just about any look you'd like... and render times are reasonable if you set you reduce bounces to zero and use direct lighting techniques (i.e. do lighting as if you were rendering in Blender Internal). That said, it's often faster to set up toon materials in Blender Internal if you're already familiar with the system. 

Furthermore, if you want Blender to generate your linework for you, Blender's Freestyle system is currently only supported with Blender Internal. However, if you want to use Cycles shaders with Freestyle, it's not that difficult (use a duplicate scene with linked data, render using BI and Freestyle, and composite the results with your Cycles scene).